[!NOTE] in 1.13+ we can use
execute unless ...instead
(This is for command blocks only. For functions, see here)
[!NOTE] You could use a redstone torch and a comparator, but it is not recomended as it causes more lag than the other methods described below.
When a command block runs, its SuccessCount tag is updated. This is the value used by comparators to decide how strong a signal to output from that command block. If the command block does not succeed, its SuccessCount will be 0.
We can use another command block to check whether the first block has SuccessCount:0 after running, so that this second block will be successful when the first block fails, then run our conditionals off of this second block. The commands would look like:
# pre 1.13
testfor @a[r=5]
testforblock ~ ~ ~1 command_block * {SuccessCount:0}
say Nobody is within 5 blocks!
# 1.13+
<any command>
execute if block ~ ~ ~1 command_block{SuccessCount:0}
say Command didn't succed
The first command is whatever you want to check wasn’t successful.
In the second command you must adjust ~ ~ ~1 command_block to match the location and type of the first block. For example, it may be ~-1 ~ ~ chain_command_block.
The third command is conditional, and is whatever you want to happen when the first command fails.
